The present invention relates to the field of athletic shoes, and, in particular, to a design made to reduce the risk of injury to the subtalar joint when an athlete in motion comes to a sudden stop.
Said injury occurs when the ankle moves from neutral (see FIG. 1,a) to the inverted (see FIG. 1,b) position and the bending torque of the external forces becomes more than the counter-resistance of the joint's ligaments.
Due to the abrupt stop, the ground reaction is applied to the lateral edge of the shoe sole. A force due to friction between the ground and the shoe (the horizontal component of said ground reaction) acts on the bottom surface of the sole in medial direction. Because the foot is placed in the shoe comparatively tightly, that action of the force of friction is transferred by the shoe to the lateral edge of the foot.
Said force of fiction provides a bending torque relative to the subtalar joint. On the other hand, the weight of the athlete's body applied to the center of mass acts vertically downward and provides an oppositely directed torque relative to the same joint. The greater the negative acceleration of the stop, the greater are the magnitudes of said controversial or oppositely directed torques, and the more probable the injury is.
To prevent said injuries, U.S. Pat. No. 3,664,040 shows a sole element having an edge band of thermoplastic material in lieu of a textile or other tape. It resists slightly, to the sliding of the foot relative to the shoe, but does not bind the foot when the body of the wearer is moving at comparatively high speed before emergency stopping.
To separate the influence of the upper and bottom surfaces of the shoe sole, U.S. Pat. No. 4,635,384 suggests a sole comprising an upper and a lower sole, communicating through the air pockets, but it does not remove a cause of a dangerous inversion, namely, decreasing of the thickness of the lateral edge of a sole during emergency stopping.
Said process is illustrated by FIG. 2, where F1 is force acting from the foot on the upper surface of the sole and on the medial shoe's quarter, and F2 is the force of friction applied from the ground to the bottom surface of the sole. Said two forces create a pair, whose action is equivalent to the torque M rotating the sole clockwise and pressing the upper lateral edge of the sole to the ground.
Moreover, when the magnitude of the torque M exceeds a certain value, it starts to rotate the sole (and the rest of the shoe with the foot inside) relative to the bottom lateral edge. It leads directly to the injury of the subtalar joint.
Implementation of the "wedge" idea can be seen in an embodiment of the U.S. Pat. No. 4,934,073, wherein a reverse wedge is shown, which lifts the ball of the wearer's foot. Since that wedge is mounted permanently and is made from resilient and flexible material, during emergency stopping it should be affected by the toque similar to M, described above, which would decrease its useful thickness. On the other hand, said wedge is placed in the sagittal plane and almost does not influence the frontal displacements of the foot.
Summarizing the existing approaches, we can see so-called "negative feedback" everywhere: the more the value of the dangerous torque M is, the worse is the capability of the athletic shoe to prevent the injury of the subtalar joint.